It is generally desirable to have bandpass filters that have at least 90% transmission in at least two passbands where they are intended to transmit light. Filters based on optical interference principles are highly versatile, and may be designed for use throughout the entire optical spectrum. For filters for communications applications, in particular, there is a need to transmit with very low losses in more than one band. The usual bands are typically: 850 nm to 950 nm; 950 nm to 1020 nm; 1060 nm to 1120 nm; 1250 nm to 1350 nm; and 1500 nm to 1650 nm.
Multi-cavity dielectric filters have been manufactured for more than 40 years. Most bandpass filters are designed to operate in transmission and will transmit one wavelength zone. The light for the adjacent wavelengths is reflected. Therefore, adjacent to the principle transmission band of a dielectric thin film filter, there is a reflection zone on each side. Outside and adjacent to the reflection zone and yet more distant from the principle transmission zone there are generally side lobes or side bands where the filter is substantially transmissive however where transmission is typically uncontrolled and quite variable.
Variation in layer thickness has been used to improve filter response as disclosed in Ser. No. 08/743,734, by the present invent or, to reduce reflections caused by mismatch between the filter material and adjoining media. Alteration is made to the thickness of layers near the input and output interfaces only in this application. In general, a bandpass filter is constructed of quarter and half wavelength layers or multiples of these units. Variation in the layer thicknesses has not otherwise been disclosed. Optimizing a second passband region outside the reflection zone has not been disclosed.
It is an object of this invention, to provide filters that have a high controlled transmission in at least a secondary passband region substantially outside the reflection zone of the filter without substantially adversely affecting transmission in the primary passband region.
It is a further object of the invention to provide an optical filter that has two passband regions separated by a reflection zone, wherein transmission within the two passband regions is at least 90%.